Marine drives such as propeller drives, surface drives, and waterjets each have many benefits for propulsion of marine vessels. However, a disadvantage inherent to all of these drives is the large amount of space taken up by the motor and the transmission, both of which are typically placed in front of the marine drive. The motor provides the power to drive the marine drive via a transmission that adapts the rotational speed of the motor to that of the marine drive. Especially in the case of an electrical motor or a gas turbine, the difference between input and output rotational speeds is quite significant and requires several speed reduction steps.
If the motor is placed over the top of a marine drive such as a waterjet (or beside or at an angle, half over, or half beside), a great deal of space in the vessel in front of the marine drive can be advantageously used for cargo or passengers, making the vessel more useful. In addition, the longitudinal center-of-gravity will be advantageously shifted aft, in general producing better vessel performance. The motor is no longer placed in the bilge, avoiding the effects of corrosion due to exposure to bilge water. If the motor is an internal combustion engine, the motor exhaust line is located relatively higher and is better protected from exhaust duct backwash.
In order to locate the motor further aft, it is possible to support the motor over the marine drive and to locate the transmission in front of the drive in the vessel with the motor output shaft protruding forward. However, this will require a special transmission and two sets of drive lines and a great deal of added weight, vessel structure and alignment labor. FIG. 1 portrays such a prior art installation, illustrating a waterjet 8 as the marine drive. It requires vessel internal support structures for motor 6, for motor mounts 1, for transmission 7 with its own mounts 2, and for forward drive carrier bearing 3. Flexible couplings 4 and drive shafts 5 are necessary to transmit power from motor 6 to waterjet 8. This is costly for a waterjet-propelled vessel builder or retrofitter to accomplish, placing him at a disadvantage in the marketplace. In addition, the installation is neither space-efficient nor lightweight.